Not Applicable.
The present application relates to a doped fiber amplifier having an increased dynamic range, and particularly for laser pump sources and a method of pumping erbium doped fiber amplifiers over a greater dynamic range.
Optical networks increasingly use wavelength division multiplexing (WDM) as a method to increase bandwidth. Multiple optical channels are combined and transmitted simultaneously as a single multiplexed signal. At the receiving end a demultiplexer separates the channels by wavelength and routes individual channels.
Optical amplifiers are commonly used in optical communication systems as in-line amplifiers for boosting signal levels to compensate for losses in a transmission link. In WDM systems, optical amplifiers are particularly useful because of their ability to amplify many optical channels simultaneously. Rare earth doped fiber optical amplifiers, such as erbium doped fiber amplifiers (EDFA) are used extensively. In addition other dopants can also be used to absorb pump energy to cause a population inversion.
In network operation, signals are periodically added or dropped for switching and routing. The number of channels, and hence the optical power of a signal may also vary due to network reconfigurations, failures or recovery from failures. In order to maintain a constant output power for each channel, the gain of the amplifier must vary with the signal power. In response to adding and dropping of signals, in particular, the signal power varies in a step function, with rapid, sometimes large changes. In order to maintain constant gain for each remaining channel, the pump power to the amplifier must be adjusted accordingly. Otherwise, with each dropped channel, the gain in the remaining channels will increase
EDFAs designed for use in WDM systems are expected to have dynamically adjustable output power levels to support the adding and dropping of wavelengths while still maintaining a constant gain and output power for each wavelength. For example, a 20 dBm (100 mW) EDFA amplifying 80 different optical wavelengths will provide an output power of 1.0 dBm (1.25 mW) for each wavelength. In a dynamic network, the EDFA is expected to keep the power per channel constant when wavelengths are added and dropped. In the extreme case, where only a single channel remains, the EDFA must reduce its output to only 1 dBm. This is a dynamic range of 19 dB.
However, pump sources for EDFAs do not have such a large dynamic range. A 980 nm semiconductor pump source has a dynamic range of 7 to 10 dB. Standard JDSU/SDLO 980 pump modules are only guaranteed to operate down to 20% of operating current IOP to provide stable output power. Below this power level grating stabilization of the lasing wavelength may not occur, or may become unstable, causing wavelength and power fluctuation. This represents a dynamic range of 7 dB. Furthermore, even if the output power were stable down to the 1.2% IOP required by the 80 channel system, the pump laser would be operating very close to the lasing threshold. This is in general an unstable operating point, as the lasing threshold is highly dependent on chip temperature.
It is highly desirable to provide an amplifier that can provide stable gain over a broad dynamic range of signal intensity for WDM systems.
It is also desired to provide a pump laser source for EDFAs which can provide stable output at very low power levels.
It is further desired to provide a pump laser source for EDFAs which has a broad range of output power.
The present invention has found that the dynamic range of an EDFA can be significantly increased by using pulse width modulation to pulse pump current at or near the minimum current stability threshold, in order to produce lower power output. The duty cycle of the pulsed current is selected to achieve a time-averaged operating condition. Thus the time averaged pump power can be reduced in a linear fashion well below the capability of a continuous wave system.
Accordingly, the present invention provides
Thus an aspect of the present invention provides
In embodiments of the invention,